Conventionally, resol-type alkylphenol-formaldehyde co-condensation resins in the form of flakes or particles have been used as resin cross-linking agents. A variety of resins have been produced as such resins, which differ in the number of carbon atoms of the alkylphenol used as a raw material. Among these, a resol-type para-octylphenol-formaldehyde co-condensation resin formed using p-octylphenol having 8 carbon atoms is suitably used as a resin cross-linking agent, because it has good compatibility with a rubber and can provide satisfactory rubber properties.
However, resol-type alkylphenol-formaldehyde co-condensation resins are generally known to have poor dispersibility in rubbers. In order to sufficiently disperse a resin cross-linking agent in a rubber, the rubber and the cross-linking agent are preferably kneaded at a temperature equal to or higher than the softening point of the resin cross-linking agent. However, if the kneading temperature becomes 120° C. or higher, the cross-linking reaction of the rubber will begin, and a uniform rubber composition cannot be obtained. As such, resin cross-linking agents having a softening point of 115° C. or lower are generally used.
Meanwhile, environmental pollution prevention has recently become a social issue, and it is necessary to reduce the amounts of impurities, residual raw materials, and residual solvents in products. Additionally, attention has also been focused on the issue of bad odors due to these residual matters, which has never been raised as a problem before. Similarly in the field of resin cross-linking agents, it is necessary to reduce the amounts of residual matters, and particularly, to reduce residual amounts of an alkylphenol used as a raw material and solvent(s) used in the production of a resin cross-linking agent. As an industrial method for reducing these residual amounts, the following method may be contemplated: a method in which the reaction time is lengthened to complete the condensation reaction of the alkylphenol and aldehyde used as raw materials as much as possible; or a method in which after the completion of the reaction, concentration is performed with heating to thereby remove the alkylphenol and the residual solvent content. With these methods, however, it is known that the softening point of the resin cross-linking agent rises, which leads to the problem of poor dispersibility in a rubber.
Japanese Patent Laying-Open No. 2006-089704 (patent document 1) and Japanese Patent Laying-Open No. 2006-063320 (patent document 2) propose adding a plasticizer or an oil to the resol-type alkylphenol-formaldehyde co-condensation resin, so as to lower the softening point of the resin cross-linking agent to improve the dispersibility in a rubber. With these methods, it is possible to eliminate the problem of poor dispersibility in a rubber caused by a rise in softening temperature, while reducing the amounts of an unreacted alkylphenol and remaining solvent(s). However, because a plasticizer or an oil is compounded with a resin, there is a concern for, for example, deterioration of the rubber properties due to bleeding of these components. The above-described methods have also had problems in that they cannot be used in the field of applications where the addition of these additives in itself is undesirable, and even in the field of applications where the addition of these additives is acceptable, if the application or customer demand varies, it is necessary to produce a resin cross-linking agent by selecting an appropriate plasticizer or oil in accordance with the demand, which makes the production operation very complicated.
Furthermore, in conventionally known resol-type alkylphenol-formaldehyde co-condensation resins, some cases have been observed where the cross-linking densities upon kneading and cross-linking with rubbers are still insufficient, and consequently, problems such as insufficient rubber properties, for example, heat aging resistance, strength, and the like, of the cross-linked rubbers, and swelling of the cross-linked rubbers have sometimes occurred.